Generally, control of the idling rotational speed of an engine of a vehicle to a predetermined level in accordance with the temperature of water or an electric load has been effected by providing bypass passageways to a throttle chamber for bypassing the throttle valve so as to regulate the volume of air flowing through the bypass passageways by utilizing the pressure differential between the inlet and the outlet of the throttle valve. However, some disadvantages are associated with this process of control. First of all, this process requires a large number of additional parts including an air regulator for use in starting the engine at low temperature, an automatic choking mechanism for use during warmup of the engine, a throw adjusting mechanism or a throttle adjusting mechanism for use after completion of engine warmup, an on-off solenoid valve for use when the cooler is turned on and a dashpot mechanism for use in deceleration of the engine. An increase in the number of parts might cause a reduction in the reliability of the system. To obviate this disadvantage, a flow rate control system has become popular which includes an electromagnetically operated valve mounted in the bypass passageways.
A flow rate control system of the last-mentioned type is shown in FIG. 1, wherein, according to this figure, an engine 10 has a suction pipe 12 and an exhaust pipe 14 connected thereto, with the suction pipe 12 mounting a fuel injection valve 15 and having a throttle chamber 16 located on its inlet end. The throttle chamber 16 mounts a throttle valve 18 in a central portion thereof and is provided with bypass passageways 20 and 22 which bypass the throttle valve 18 and constitute an inlet of an inflow passageway and an outlet of an outflow passageway, respectively, of a flow rate control system.
Located on the upstream side of the throttle chamber 16 is an air flow meter 24 which measures the volume of air flowing through an air cleaner 26 by converting the opening of a vane 28, which is varied in accordance with the air volume, to an electric output by means of a potentiometer 30.
The suction and exhaust pipes 12 and 14 are maintained in communication with each other through an exhaust gas recycling passageway 32, to enable a portion of exhaust gases to be returned to suction pipe 12 through an exhaust gas recycling valve 34 depending on engine operation condition. The engine 10 has mounted therein a crank angle sensor 35 for sensing the revolving velocity of the engine 10, and a water temperature sensor 36 for sensing the temperature of cooling water. A flow rate control system 38 is secured to the bypass passageways 20 and 22 of the throttle chamber 16.
The flow rate control system 38 comprises a flow rate adjusting section 40 and a solenoid section 42. The solenoid section 42 drives the flow rate adjusting section 40 to thereby effect control of the flow rate of a fluid (air). More specifically, the flow rate adjusting section 40 includes a body 44, an outlet passageway 46 formed in a central portion of the body 44, and an inlet passageway 48 located around the outlet passageway 46 substantially in the form of an inverted letter U and separated therefrom by a partition wall 52. The flow rate adjusting section 40 further comprises a rod 50 extending transversely through the outlet passageway 46 and inlet passageway 48 and having secured thereto a first valve body 58 and a second valve body 60 adapted to engage valve seats 54 and 56 respectively which are formed in the partition wall 52. The rod 50 is urged by the biasing force of a spring 62 through the second valve body 60 to move forwardly (leftwardly in FIG. 1), and penetrates at its forward end a core 64 of the solenoid section 42. The rod 50 has a plunger 66 secured to its forward end, and the core 64 and the plunger 66 are juxtaposed against each other at surfaces thereof which are conical in shape. A cylindrical coil 68 located around the core 64 and plunger 66 generates a magnetic force to cause them to be attracted to each other, so as to cause the rod 50 to shift rightwardly in FIG. 1 against the biasing force of the spring 62.
The coil 58 is controlled by, for example, an operation circuit 70 of a micro-computer, which processes an air volume signal produced by the potentiometer 30, an engine rpm signal produced by the crank angle sensor 34 and a cooling water temperature signal produced by the water temperature sensor 36 and supplies a predetermined output signal. The operation circuit 70 is also operative to control the fuel injection valve 15 to vary the amount of fuel injected in accordance with the amount of air flowing through the flow rate control system 38, to provide a predetermined engine rpm.
As described hereinabove, the flow rate control system 38 continuously effects control of air volume in accordance with the engine rpm, water temperature, etc., that have been sensed, to thereby automatically keep the engine rpm at a predetermined level. This system suffers the disadvantage that, as shown in solid line in FIG. 2 a hunting phenomenon might occur. This phenomenon would be marked when the diameter of the first valve body 58 is smaller than that of the second valve body 60. An investigation into the cause of the phenomenon has revealed that, since the flow rate control system 38 effects continuous control of air volume in place of the on-off control effected in the prior art, the hunting phenomenon occurs when the first valve body 58 and second valve body 60 are slightly released from contact with the valve seats 54 and 56, respectively, or when an input of small magnitude is fed into the flow rate control system 38 (at a point a in FIG. 2). When the valve bodies 58 and 60 are slightly released from contact with the respective valve seats 54 and 56, pressure differential of large magnitude would occur between the inlet and outlet of each of the valve seats 54 and 56, causing a sudden change to occur in the flow air and producing, at the same time, a pressure difference between the first valve body 58 and second valve body 60.
Proposals have been made to avoid this disadvantage by rendering the diameters of openings of the valve seats 54 and 56 equal to each other as shown in FIG. 3, to thereby equalize the crosssectional areas of channels for the fluid to flow in passing through the valve assembly 58, 54 and 60, 56. However, even with these proposals the following disadvantages would be inevitable.
If the diameters of the openings of the valve seats 54 and 56 are equal to each other then it would be impossible to assemble the flow rate control system in a condition in which the valve bodies 58 and 60 are secured to the rod 50, so that a reduction in reliability and a rise in cost might be caused by an increase in the number of parts and the number of process steps to be followed in fabrication.
Moreover, even if an attempt is made to equalize pressure differentials F.sub.1 and F.sub.2 between the valve bodies 58 and 60 when a static condition prevails on the side of the inlet passageway 48 and the side of the passageway 46 by equalizing the diameters of openings of the valve seats 54 and 56, it would be impossible to equalize the pressure differentials F.sub.1 and F.sub.2 because channels b and c of two directions, indicated by arrows in FIG. 3, would have flow coefficients distinct from each other due to a difference in the length of the channels and other factors.
Furthermore, even if it were possible to equalize the pressure differentials F.sub.1 and F.sub.2, it would be impossible to obtain a uniform flow condition when the valve bodies 58 and 60 are released from contact with the respective valve seats 54 and 56, inevitably resulting in the pressure differentials F.sub.1 and F.sub.2 becoming unbalanced.